Life history variation and environmental fluctuations can influence species' abilities to stably coexist in a particular habitat. For instance, dormancy behavior and nonlinear environmental responses should permit a species to thrive where it would otherwise be unable to recover from low densities. Using two widely co-occurring and ecologically-similar species of freshwater duckweed plants (Spirodela polyrhiza and Lemna minor), we tested the hypotheses that facultative dormancy, negative frequency-dependent growth rates, and nonlinear environmental responses allow duckweed species to stably coexist. Using the results of controlled competition experiments across a range of genotypic and environmental backgrounds, we developed a general model for duckweed competition. By adjusting this model's empirically-derived formulation and parameters, and testing for reciprocal invasibility across a range of environmental conditions, we evaluated whether coexistence mechanisms acted in concert or were redundant in their effects on duckweed coexistence.
Results/Conclusions
Using experimentally-derived estimates of vital rates and competitive interactions, we identified one mechanism — negative frequency-dependent growth — acting to maintain duckweed coexistence in static temperatures. Under fluctuating temperatures, two additional coexistence-promoting mechanisms became evident — interspecific variation in thermal reaction norms and facultative dormancy behavior at low temperatures. Paradoxically, we found that investment in dormancy by the competitive subordinate (Spirodela) did not necessarily facilitate coexistence, and constrained both its own and its competitor's abilities to invade a community. Furthermore, when fluctuation-dependent mechanisms were experimentally removed, negative frequency-dependent growth compensated for the loss and maintained coexistence, albeit at relatively low levels. Conversely, when the fluctuation-independent mechanism (negative frequency-dependent growth) was removed, coexistence of Spirodela (the weaker competitor) became nearly impossible, despite the presence of fluctuation-dependent mechanisms. These results illustrate how relatively minor differences between ecologically-similar taxa can interact with one another leading to relatively complex coexistence patterns across environmental gradients. Our approach may also be useful for identifying the optimal environmental state and timing for a species' successful invasion into new communities.